Voxel-by-voxel comparison of gray matter density revealed significant reductions in regional gray matter density within left posterior STG in schizophrenia patients but not in affective disorder patients in comparison with control subjects, in agreement with our ROI finding of smaller left posterior STG gray matter volume in schizophrenia compared with both controls and the affective psychosis group Hirayasu et al. (1998)
. Reduced STG gray matter volume in schizophrenia has been among the most consistent of all structural MRI findings, with all 12 MRI studies concurring (reviewed in Shenton et al., 2001
, McCarley et al., 1999
, Pearlson, 1997
). When P
values were adjusted to take into account the spatial extent of voxel clusters, schizophrenia subjects showed decreased gray matter density compared with normal controls in regions previously described to be abnormal in at least some manual ROI analyses, although not always with the same laterality or subdivision. These regions included (see reviews in Shenton et al., 2001
, McCarley et al., 1999
) (1) right dorsolateral prefrontal cortex (DLPFC), where volume reductions in schizophrenia subjects have been reported in a majority of structural studies (30/50); (2) right inferior parietal lobule (IPL), where ROI studies have shown smaller gray matter volumes (9/15 studies of parietal lobe, especially of IPL); (3) left and right anterior cingulate gyrus (ACG); and (4) left and right insula. The few manual ROI studies of these last two regions reported smaller volumes in schizophrenia for both ACG (Szeszko et al., 2000
) and insula (Crespo-Facorro et al., 2000
With respect to these regions in patients with affective disorder compared with control subjects, the left and right insula were the only regions to show a decrease in gray matter density. This result has not been described before and may suggest that the insula abnormality is present in affective disorder as well as in schizophrenia. On the other hand, the left STG density decrease appears specific to schizophrenia. This left STG result is entirely consistent with the manual ROI findings of (Hirayasu et al., 1998
), who did not investigate the insula.
To provide a rigorous statistical test of the validity of the SPM cluster analysis, we conducted a permutation analysis, randomly assigning subjects to different groups (controls and schizophrenics). This analysis, in contrast to cluster analysis by SPM, requires no distibutional assumptions (such as normality) about the data. We used more permutations than other studies using this technique (Sowell et al., 2000
; Bullmore et al., 1999
), which, together with the consistent results using both 225 and 250 permutations, suggests that our procedure provided a stable and valid estimate of P
Motivated by the positive manual ROI findings in medial temporal lobe, we also used a smaller smoothing kernel and SVC and found density reduction in the left medial temporal lobe in the schizophrenia group, consistent with manual ROI analyses, both of the present data set (Hirayasu et al., 1998
) and of other sets (Shenton et al., 2001
; McCarley et al., 1999
). This suggests the importance of smoothing kernel size and small volume correction in evaluation of small regions of interest, a finding underscored by a recent study where a small smoothing kernel and SVC for SPM and manual ROI analysis both showed increased hippocampal densities and volumes in taxi drivers when compared to those of normal controls (Maguire et al., 2000
). The present VBM analysis found statistically significant medial temporal gray matter density reduction in schizophrenia compared with affective psychosis, whereas Hirayasu et al. (1998)
found smaller, but not statistically significantly different, manual ROI volumes in schizophrenia compared with affective psychosis. This fact further supports the idea that, as stated in this Discussion, factors other than the volume alone (i.e., shape) influence the VBM results and further confirms the importance of comparisons with manual ROI studies as well as careful interpretation of the results.
Our interest in following up on the VBM analysis has led us to implement an ROI analysis on the insula region that showed density reduction in the current study. To check the hypothesis that the insula volumes are reduced bilaterally in schizophrenics, when compared to normal controls, we studied a subsample of 12 subjects and 12 schizophrenics, and the results were consistent with the VBM results. That is, the schizophrenia group was characterized by insula volume de-crease on both left (P(24) = 0.002) and right (P(24) = 0.001, independent sample t test) sides. However, manual ROI analysis of the affective psychosis sample (n = 12) showed close to significant, but not significant differences between controls and affectves on the right (P(23) = 0.071) and no significance on the left (P(23) = 0.222). In addition, insula volumes in FE schizophrenics differed from FE affectives on the left (P(23) = 0.010) and the right (P(23) = 0.037) sides.
The first study comparing manual ROI with VBM in schizophrenia (Wright et al., 1999
) evaluated two chronic schizophrenia samples, neither of which showed any gray matter deficits in previous ROI analyses (despite measuring whole brain volume, temporal lobe, and planum temporale volumes (Sharma et al., 1998
; Frangou et al., 1997
)). The VBM study revealed gray matter deficits located in insula, cingulate gyrus, temporal pole, middle temporal gyrus, and inferior frontal gyrus (note overlap with the present firstepisode study in insula and cingulate gyrus). The reasons for the mismatches between their ROI and VBM methods in temporal lobe ROI were not explained and were not further investigated. Of note, however, was a recent VBM study reporting regional gray matter deficits in superior and medial temporal gyrus, insula, and ACG (Sigmudsson et al., 2001
); this study used normalization to the subjects’ brain template with no smoothing and did not provide manual ROI validation.
The differences between these studies’ results, taken together with the present study, further stress the necessity for validation of voxel-based analyses by ROI studies (see discussion of VBM and other automated analyses in McCarley (2001)
). Because the validity of VBM has not yet been systematically studied, the sources of differences between VBM and ROI methods remain uncertain. False positive or false negative VBM findings might arise from the changes in the shape or displacement of structures in the course of spatial normalization. Specifically, as the VBM does not tend to match perfectly every possible structure, as some of the other sophisticated methods of registration do (e.g., Fishl et al
., 2000; Thompson et al
., 2001; Narr et al
., 2001), the output of the statistical analysis gives information about the local volume differences. This information is prone to some systematical biases, which could arise from not only the errors caused by the misregistration of anatomical structures, but also movement or gray and white matter intensity patterns that differ between groups (see Bookstein, 2001
, and Ashburner and Friston, 2001
, discussion in NeuroImage). Recently, methods that attempt to minimize the biases introduced in the analysis by the volume changes during the registration procedure were introduced (Ashburner, 1998; Good et al
., 2001). These methods attempt to adjust, or modulate, the voxel values according to the Jacobian determinants derived from the spatial normalization step, thus allowing for analyzing absolute volume (optimized voxel-based morphometry (Good et al
., 2001)), relative position of the brain structures (deformation based morphometry), and shape differences (tensor based morphometry) (Ashburner and Friston, 2001
). Moreover, some of these potential confounds are not only limited to VBM, but apply also to shape analysis and can be a factor even when perfect warping is performed. As VBM is much faster than these techniques, it makes it possible to obtain reproducible results by studying much bigger populations.
VBM has only recently been thought of as a possible replacement for manual ROI analyses (the current gold standard), but it appears to have several potential advantages. For example, VBM enables regional comparisons throughout the whole brain without restriction to a few selected areas in the typical manual ROI methodology. Indeed, in the present study, VBM pointed to first-episode schizophrenia gray matter differences in regions not examined in our ROI study. Other advantages are the reduction of labor and the ability to use large samples with an attendant increase in statisticalpower. These advantages appear important in disorders such as schizophrenia that have subtle structural changes in several brain regions and many heterogeneous clinical subtypes. The latter consideration would also suggest the importance of studying more homogeneous, larger patient populations, which can be more easily accomplished using VBM analyses.
Despite all these advantages, our study showed that VBM methodology should be chosen with caution, as different hypotheses might require slightly different approaches and parameters. Our data also suggest that negative findings in the VBM analysis do not necessarily preclude manual ROI differences, so the VBM results should be interpreted with caution. A clear example was our need to use a smaller smoothing kernel and SVC to detect hippocampal differences. Had manual ROI results not been available to suggest this analysis, we would not have used it.
Thus, as a general rule, we suggest that each VBM study should be compared with manual ROI analysis until validity is established. We further suggest, since VBM findings point to brain areas not yet systematically studied with ROI analysis, that VBM could be profitably used in an exploratory manner to point to brain regions that might subsequently be evaluated using manual ROI analyses. The example of this approach has been demonstrated on the insula case. Follow up manual ROI analysis was carried out, and similar, although not identical, results to the VBM analysis were obtained—see discussion above.
With respect to the first-episode schizophrenia population, the current VBM data are suggestive of gray matter deficits in a number of regions, although the relatively small sample places limitations on generalizability. These data, derived from patients early in the course of their illness, are compatible with developmental hypotheses of schizophrenic abnormalities. It will be important to follow up these subjects so as to determine if the deficits progress and are thus compatible with a still controversial theory of schizophrenia which posits developmental abnormalities followed by a later neurodegenerative process (see Mednick and McNeil, 1968
; McCarley et al., 1996
; and reviews in Shenton et al. (2001)
and McCarley et al. (1999)
These VBM data reinforce the concept (Shenton et al., 2001
; McCarley et al., 1999
) that gray matter abnormalities in schizophrenia are not diffuse, equally distributed in all regions, but rather are concentrated in particular regions. Of these regions, the superior temporal gyrus, and especially on the left (dominant) side, appears to be strongly and frequently affected in schizophrenia.